This invention relates generally to the oxygenation of water and more particularly to a method and apparatus employing packed column techniques to increase the concentration of dissolved oxygen in an aquaculture system.
In recent years, aquaculture production has played an increasingly prominent role in the commercial fish industry, both in the United States and worldwide. The controlled cultivation of fish and other aquatic products is predicted to increase even more dramatically in the near future due in large part to the recognition that the capacity of commercial capture fisheries of traditional species has reached or soon will reach its limit. It is estimated that worldwide aquaculture production will increase five fold by the year 2000 to 21 billion pounds annually. At present, total aquaculture production in the United States is estimated at 500 million pounds and accounts for approximately 11% of the total edible fish and shellfish consumed nationwide. Both private industry and governments have recognized the growing contribution that aquaculture will continue to make to the national and world economy.
The aquaculture systems that are currently in operation make use of a wide variety of facilities having different components, depending upon the species of aquatic organism that is involved. In each aquaculture system, the most economical type and configuration of the rearing facility is determined in part by the species involved and in part by other factors. However, it is necessary in all systems to make water borne oxygen available for the metabolic processes of the aquatic organisms in order to maintain the health of the species and to increase production. Oxygen is normally dissolved by natural interaction between the air and water and by photosynthetic plants and natural infringements on the water such as rain. This maintains an acceptable concentration of oxygen in the water when there is a balance between the aquatic organisms and the natural systems. However, when aquatic plants and animals are present in the water in abnormal concentrations as is the case in high production aquaculture systems, it is necessary to artificially increase the oxygen content of the water.
In the past, the mechanical aerators that have been used for this purpose have taken the form of turbulent bubbling devices, "egg beater" threshers, water wheel churners, or waterjet pump systems. All of these types of aerators are expensive to construct and operate and are inefficient in transferring oxygen to the water, especially when the oxygen level is already high. The closer the oxygen concentration is to saturation, the more difficult and costly it becomes to increase the oxygen level in the water. Perhaps even more importantly, the addition of air to water increases the level of dissolved nitrogen and other gases as well as the oxygen level. When nitrogen is present in water in above normal concentrations, it can cause harmful effects. For example, fish can develop nitrogen bubbles in the bloodstream, and the resulting discomfort can reduce the productive biomass transfer and can lead to other adverse consequences. Because nitrogen is much more soluble in water than oxygen, aeration increases the nitrogen content of water to a greater extent than the oxygen content, and the water can easily become supersaturated with nitrogen. Devices which apply bubbles of oxygen and other gases to water are characterized by inefficiency in the transfer of oxygen to the water and thus common excessive quantities of gas.